1. Field of the Invention
The invention relates to a joint for conduit of a wall thickness sufficient to permit threads to be formed therein, and, more particularly, to a joint wherein sections are assembled by using a coupling ring which will permit sections to swivel with respect to one another during coupling.
2. Prior Art
Until very recently, there were on the market only two kinds of steel conduit for electrical cables; one of these was called rigid pipe and the other was called light walled conduit.
The rigid pipe, a thick walled pipe, has been available for many years and conventially is used for the transmission of gas and water. It also is used for sheathing electric wires in buildings in which case it is frequently embedded in concrete. This type of pipe comes in various lengths, depending upon the trade but is essentially from ten feet to twenty feet long.
Rigid pipe sections are connected by screw couplings. Both ends of a pipe section have tapered male threads formed thereon. These threads are V-shaped to standardized dimensions having a slightly flattened crest and a slightly flattened base. Both ends of a screw coupling have tapered female pipe threads which mesh with the male pipe threads. The mesh is quite good and, because the threads are tapered, when they are joined they make a joint that is essentially hermetic, that is, a joint that does not require sealing agents such as mastic or pipe compound, although sealing agents may be used.
The difficulties associated with the use of rigid pipe become apparent when such pipe is to act as a conduit for electric cables. Because the pipe is thick walled, each section is highly priced. In addition, each section is difficult to handle because of its weight. Because both ends of the pipe are threaded, each section must be rotated in the field when it is connected to another length of pipe. The weight of the pipe section makes the rotation difficult.
Moreover, since the screw couplings must mesh with male threads on adjacent pipe sections, the sections must be in perfect axial alignment or the threads will not mesh properly. As is often the case in field installations, or where replacements must be made to existing pipe systems, the pipe sections are not in perfect alignment and problems result in assembling the sections.
The other kind of conduit which has been developed for sheathing electrical cables is light walled conduit. Light walled conduit was designed solely as a conduit for encasing electric cables to protect them from damage. Because it does not have to take large stresses, it is thin walled. Obviously, an advantage of light walled conduit is its lighter weight and its lower cost.
A disadvantage is that it cannot be used in instances where it might be subjected to heavy loads. In addition, because of its thin wall, this type of conduit can not be threaded. Compression couplings are used most often to join sections of light walled conduit. The ensuing joint cannot withstand substantial internal pressure or tensile stress. The advantage of compression couplings over the screw couplings used in conjunction with the rigid pipe is that if any section of tubing or any fitting is damaged, the coupling can be opened and the damaged pipe section or fitting can be pulled out transversely without removing the fittings or sections that were no damaged.
Recently, due to the scarcity of and resulting higher prices for steel, a medium walled pipe known as utility pipe or intermediate metal conduit was developed having a wall thickness less than the wall thickness of the rigid pipe but greater than the wall thickness of the light walled conduit. This intermediate metal conduit is heavy enough to take intensive loads and can be hermetically sealed at the joints.
Intermediate metal conduit has used screw couplings for joints, causing the same problems to arise in connection with this conduit as with the rigid pipe. Namely, the necessity of rotating conduit sections to make up joints. Further, the axial alignment problem inherent in the rigid pipe system is also present.
It is toward elimination of the foregoing and other difficulties that the present invention is directed.